Inflatables are used downhole for a variety of applications. Typically a valve that opens responsive to applied pressure regulates the inflatable. In some applications there can be a series of valves. In either case, the valve assembly is sensitive to solids that may be present in well fluids. One example is in high-pressure wells where very dense mud formulations are required for pressure control. In these instances the mud contains a significant amount of grit that can undermine the operation of the valves on the inflatable. Another problem is settlement of solids. If solids in appreciable quantities get into the inflatable and settle out, then the inflatable will not go back down sufficiently when deflated and will present problems when it is time to retrieve it. Mixing of fill fluids and well fluids could cause a reaction that creates a very viscous emulsion or worse, a resulting compound that hardens inside the inflatable, making subsequent extraction of the inflatable difficult, if not impossible.
In the past, attempts were made to overcome this problem. One technique involved putting clean fluid, such as water above the inflatable and topping it off with a large plug of grease. The idea was that the grease plug would remain cohesive and displace the clean fluid into the inflatable, without allowing the grit laden well fluids an opportunity to reach the delicate valve system on the inflatable. This system was unreliable as the downhole temperatures may reduce the grease viscosity and allowed well fluids to get by the plug and into the inflatable. Another attempt involved a downhole pump powered with a wireline. The pump was disposed in a clean fluid and the wireline extended through a movable barrier with the well fluids on top. As the pump delivered the clean fluid to the inflatable, the well fluid acted on the barrier to push it downhole along the electric line. The problem was that the electric line had an irregular outer surface and it was difficult in some applications to obtain a good seal between the stationary electric line and the bushings in the movable barrier. Apart from that the arrangement required special equipment to run the electric line to power the pump. There were cost and physical space concerns involved in the deployment of this arrangement. Efforts to provide a regular outer surface on the electric line through the use of a smooth coating were successful in some applications but were abandoned as being too costly and unreliable in the elevated temperatures in some existing deep well applications.
Accordingly, what was needed was a simple system to separate fluids to ensure reliable inflatable operation. At the same time, the system had to accommodate running in while circulating or allowing equalization of well fluids into the string during run-in. The system would also need to allow fluid communication from inside to outside the string during retrieval to prevent pulling out a wet string, laden with the weight of well fluids internally. Ideally, the system would be simple to construct and to operate, using familiar components as much as possible and requiring no specialty equipment at the surface or complicated procedures that would necessitate specialty service company personnel or unique training for the rig hands. These benefits in a variety of combinations are some of the beneficial aspects of the present invention. These and other features of the apparatus and the methods of the present invention will be more readily appreciated by a review of the description of the preferred embodiment, which appears below.